This invention relates to alkylation and transalkylation processes and a catalyst for use therein. The invention is particularly concerned with a catalyst which, when used in alkylation and transalkylation processes, minimizes side reactions, such as cracking and cyclization, that lead to undesired by-products and therefore has an improved selectivity for the desired product.
In the past it has been common practice to alkylate aromatic molecules such as benzene, toluene and xylene with ethylene, propylene and other olefins using acidic homogeneous Friedel-Crafts type catalysts such as aluminum halides or heterogeneous acidic silica-alumina catalysts. Such processes have several disadvantages including corrosion problems caused by some of the catalysts and difficulty in controlling the product distribution obtained from the alkylation reactions. Often, the desired product is the monoalkylate rather than the di- or trialkylate. In an effort to avoid a large production of di- and trialkylate products and to extend catalyst life, it is conventional practice to use a large excess of the aromatic compound.
To avoid some of the problems associated with earlier commercial alkylation processes, solid zeolite-containing catalysts have been used in recent years to promote the alkylation of aromatic compounds with olefins and other alkylating agents, especially the alkylation of benzene with ethylene. Such zeolite-containing catalysts are normally prepared by combining a zeolite with a refractory oxide binder or precursor thereof, mulling and extruding the mixture, drying the extrudates and then calcining the dried extrudates at high temperatures to provide the extrudates with the strength required to withstand commercial operations. Naturally occurring and synthetic zeolites typically contain a relatively large concentration of sodium ions and are therefore not catalytically active. Thus, before a zeolite is mixed with the refractory oxide component or precursor thereof in the manufacturing of a zeolite-based catalyst, the zeolite is normally subjected to ion exchange, typically with ammonium ions, to reduce its sodium concentration as low as practically possible and increase its catalystic activity. However, since ammonia is known to poison the acid sites of the zeolite, it is common practice to carry out the calcination of the dried extrudates at such temperatures that substantially all of the ammonium ions in the catalyst are decomposed into hydrogen ions and ammonia which is driven out of the catalyst as a gas. Often, such catalysts will contain less than 50 ppmw ammonium ions, calculated as (NH.sub.4).sub.2 O on a volatiles-free basis.
Normally, zeolite-based alkylation catalysts prepared as described above are used in fixed bed reactors through which the reactants are continuously passed. Although such fixed bed processes using zeolite-containing catalysts have advantages over earlier commercial processes, the selectivity for monoalkylation, especially when producing cumene by reacting propylene with benzene, has been observed in pilot plant studies to vary from one catalyst batch to another with some of the selectivities being so low that impurities appear in the product stream in concentrations large enough to dictate the use of additional equipment or process modifications to reduce the impurity level.
Accordingly, it is one of the objects of the present invention to provide a catalyst containing a zeolitic or nonzeolitic molecular sieve, and a method for preparing such a catalyst, that has high selectivities for the desired product when used to catalyze alkylation and transalkylation reactions, which selectivities do not substantially vary from one batch of catalyst to another. This and other objects of the invention will become more apparent in view of the following description of the invention.